1. Field of the Invention
This invention relates to a device that stores variable length packets in fixed length packets. In particular, it relates to a device that multiplexes AAL Type 2 short cells on an ATM connection.
2. Description of the Related Art
In recent years, various types of information are being transferred via networks. However, the transmission rate and quality (delay time, error rate, etc.) required for each type of communication service are different. For example, when audio data are transferred, a slow transmission rate is sufficient, but it is necessary to keep the delay time as short as possible. When motion picture data are transferred, fast transmission rate and short delay time are required at the same time. Meanwhile, when E-mail and files are transferred, or when control signals are transferred, the requirement regarding delay time is not so strict. ATM has become a widespread technology for handling a unified communication service that provides various transmission rates and qualities as described above.
In ATM, information is transferred stored in fixed length ATM cells. An ATM cell consists of a 5-byte header and a 48-byte payload. Routing information, etc. is stored in the header, while the information to be transferred is stored in the payload.
FIG. 1 is a configuration diagram of a CLAD. A CLAD (CeLl Assembly and Disassembly unit) has a function that stores information to be transferred in ATM cells and a function that extracts information stored in the ATM cells and reproduces the transferred information.
FIG. 1 shows the cell assembly section of a CLAD. This CLAD consists of a data storage buffer 501, a data amount monitoring section 502, a readout control section 503 and an ATM cell header assembly section 504. Input data are stored in the data storage buffer 501 in the order in which they arrive. The data amount monitoring section 502 monitors the amount of data stored in the data storage buffer 501; when enough data to make a cell (for example 48 bytes) have accumulated, that fact is posted to the readout control section 503. When the readout control section 503 receives this notice, it reads out enough data for 1 cell from the data storage buffer 501; then the ATM cell header assembly section 504 appends a header to the data that were read out to generate a cell and the cell is sent out.
In communication using ATM, normally the transferred information is stored in ATM cell payloads for the transfer. A connection that transfers ATM cells is identified by VPI/VCI. One ATM cell stores information that is transmitted via one connection.
However, in mobile communication using a wireless network, in order to use the communication band effectively the information to be transferred is compressed; for example, the information might be transferred at the low rate of several kb/s to several tens of kb/s. When information to be transferred at such a low rate is assembled into ATM cells, the cell assembly delay becomes large, which is undesirable from the point of view of quality. That is to say, in a general ATM, as explained above, the information to be transferred is assembled into an ATM cell only when a certain amount of information has accumulated, so if that information is transferred at a low rate, the interval between sending of ATM cells becomes long, which sometimes causes delay.
At present, in, for example, an ATM forum or ITU-T, in order to reduce delays such as described above work has been proceeding to make it possible to store variable length packets called short cells, containing only small amounts of information, in ATM cells.
FIG. 2 conceptually shows the processing in which short cells are stored in the payload of an ATM cell. A short cell consists of a short cell header and a short cell payload. A short cell header includes a CID (short cell Connection IDentifier) for the purpose of identifying the connection of that short cell, and an LI (Length Indicator) that indicates the payload length of that short cell.
The payload of each ATM cell is stored in one or more short cells. Each ATM cell can store short cells that have different CIDs from one another. Storing short cells that have mutually different CIDs in one ATM cell in this manner, that is to say, sending short cells having mutually different CIDs on one ATM connection, is called "short cell multiplexing". Depending on the length of a short cell and the location where that short cell is stored, it is also possible for one short cell to be split and stored in 2 ATM cells. This condition is called "overlap".
By multiplexing a plurality of short cells on one ATM connection in this manner, it is expected to become possible to transmit information at a low bit rate with small delay and to make efficient use of the ATM connection.
In order to construct a system that stores and transfers short cells in ATM cells, basically it is sufficient to install a CLAD shown in FIG. 1 in each Virtual Connection (VC) in the ATM network. For example, if this kind of system is used in a mobile communication network, in a base station (local area station) that sends and receives wireless signals between mobile terminals a CLAD is installed in each virtual connection that is established between that base station and the exchange station (ATM switch) that accommodates that base station. The base station generates short cells from the information received from the mobile terminal, then uses the CLAD to store that short cell in an ATM cell and sends that ATM cell to the exchange station via an ATM connection.
In general, in a system such as a mobile communication network, there are many base stations. Consequently, in order to hold the cost of the system down, the cost of each of the base stations must be reduced.
However, if a CLAD is installed in each ATM connection in each base station as described above, the hardware configuration will become large-scale, and it will become difficult to hold the cost down. In order to solve this problem, it is desirable to have a configuration in which short cells can be multiplexed on a plurality of ATM connections in one device.
In addition, each short cell is required to have different quality. Accordingly, the quality of the ATM connection that transmits an ATM cell that stores short cells should be controlled in a manner that is related to the quality of the short cells that are being transmitted. However, at present technology for multiplexing short cells on an ATM connection is still under development and has not yet been established. For this reason, technology for controlling the quality of an ATM connection considering the quality of each short cell also has not been established.